The present invention, shown schematically in FIG. 1, is a re-circulation system employing cyclones, and belongs to a class of equipment used for de-dusting and dry-gas cleaning.
As a matter of fact, cyclones are de-dusters used in many types of industries with two purposes: removal of particulate matter emitted from processes, before release to an atmosphere (pollution control and/or raw material recovery); or as reactors for removal of acid components from flue gases by dry injection of appropriate sorbents. These reactors are frequently followed by bag filters for fine particle recovery.
Industrial cyclones vary in size and shape, with the most common being of the reverse-flow type.
The first reverse-flow cyclones date from the 19th century, and their design has evolved mostly from empirical observation.
Theoretically, cyclone efficiency increases with gas flow rate, but in practice there is a limit beyond which efficiency decreases. This is due to saltation or re-entertainement (Licht, 1980), much like what happens in sand dunes which are blown by a strong wind.
To remedy this problem, partial gas re-circulation has been proposed, using a fan or appropriate ejector 3xe2x80x2 in combination with a reverse-flow cyclone 1xe2x80x2 (FIG. 2, Berezowski and Warmuzinski, 1993). Similar examples may be observed in U.S. Pat. No. 3,254,478.
To increase cyclone efficiency, cyclones may be connected in series, as long as correctly designed, but with a cost of increased pressure drop and operating costs (Salcedo, 1993).
Thus, cyclone re-circulation systems were developed, which included a straight-through cyclone (from now on referred to as a concentrator) upstream from a reverse-flow cyclone (from now on referred as a collector), with partial re-circulation from the collector to the concentrator, using some fan. Such a system is schematically shown in FIG. 3 (Crawford, 1976; Svarovsky, 1981; Wysk et al., 1993). This system is disclosed in U.S. Pat. No. 5,180,486. Gas to be treated enters the concentrator 2xe2x80x3 through a tangential entry, rises in a vortex flow and is divided in two parts: one that escapes to an atmosphere and the other that enters the collector 1xe2x80x3, also through a tangential entry. Here the gas follows a descending vortex, until it changes direction due to an established pressure field (thus the name of reverse-flow) and exits from a top of the collector via a cylindrical tube, a vortex finder, of some appropriate length. As they follow the descending vortex, solid particles are thrown against a wall of the collector due to centrifugal forces, and then fall to a bottom of the collector, thereby being separated from the gas. The gas and remaining particles exiting the collector are re-cycled to the concentrator via a centrifugal fan 3xe2x80x3.
These systems may be much more efficient than single reverse-flow cyclones (collectors), and their collection efficiency is given by:                     η        =                                            η              con                        ⁢                          xe2x80x83                        ⁢                          η              col                                            1            -                          η              con                        +                                          η                con                            ⁢                              xe2x80x83                            ⁢                              η                col                                                                        (        1        )            
where xcex7con and xcex7col are concentrator and collector efficiencies, respectively. This equation shows that for xcex7conxe2x89xa6xcex7col, system efficiency is always lower than that for a singe collector (xcex7col), but that for xcex7con greater than xcex7col, system efficiency is always larger than that for a single collector. Thus, these systems are only interesting whenever concentrator efficiency is significantly higher than collector efficiency. This concept is schematically shown in FIG. 4.
Summing up, there are in the marketplace cyclone re-circulation systems that may be, under some circumstances, significantly more efficient than single reverse-flow cyclones, which use a concentrator upstream from a collector, with re-circulation from the collector to the concentrator through an appropriate fan or ejector. However, as shown, they are not always more efficient than single collectors.
There are also gas cleaning devices that employ dry sorbent injection of finely divided powders, but they still have high investment costs (Carminati et al., 1986; Heap, 1996; Fonseca et al., 1998).
The present invention has as a main objective to increase collection efficiency of cyclone dedusters with re-circulation, even when concentrator efficiency drops below collector efficiency.
It is also an objective of the present invention to make available a highly efficient system for de-dusting and acid gas cleaning of flue gases.
Additional objectives will become obvious following the remaining description and from the proposed claims.
The proposed objectives are achieved by considering a system of re-circulation cyclones, where a collector is located upstream of a concentrator, and re-cycling is performed by an appropriate fan, venturi or ejector.
With an objective of obtaining cyclone systems which are more efficient than those available in the marketplace, but with similar investment and operating costs, which may be used at high temperatures and pressures or for dry gas cleaning, a study has been initially made with regard to a most efficient configuration.
It is verified that, although system components are essentially those from the prior art, inverting their relative position makes a resulting system always more efficient than single reverse-flow cyclones or than re-circulation systems with a concentrator located upstream from a collector. Because a concentrator and collector are in series, investment and operating costs are similar to those associated with re-circulation systems with a collector downstream from a concentrator. Employing a venturi for re-circulation makes it possible to use this resulting system at very high temperatures ( greater than 1000xc2x0 C.). For larger flow rates, appropriate fans or ejectors may be used. These systems may also be used for acid dry gas cleaning, since reverse-flow cyclones may be excellent reactors for this purpose.
By simple theoretical arguments, a solution to this problem is a system where a collector is located upstream from a concentrator. Global efficiency for this system is given by:                     η        =                              η            col                                1            -                          η              con                        +                                          η                con                            ⁢                              xe2x80x83                            ⁢                              η                col                                                                        (        2        )            
Because the denominators of equations (1) and (2) are the same, and because the numerator of equation (2) is always larger than that of equation (1), efficiency of the resulting system is always higher than that of re-circulation systems available in the marketplace. This concept is shown in FIG. 5.
As previously stated, besides of the resulting system efficiency being always larger than that of the prior art system as shown in FIG. 3, where the concentrator is located upstream from the collector, for comparable geometries and sizesxe2x80x94as was previously seen by comparing equations (1) and (2) and also by comparing FIGS. 4 and 5xe2x80x94the resulting system has an efficiency always larger than that of a single collector, unlike what happens whenever a concentrator is located upstream from a collector, as referred to above.
The resulting system may also be used in advantage over existing reactors for dry gas cleaning (spray dryers or venturi scrubbers) and for acid gas cleaning (HCl, HF, SO2 and NOx), where very compact and high efficiency units may be designed.